Quote from: knightcrusader on May 28, 2017, 01:13:33 am
Just wanted to point this out incase other people missed it, but after lots of research into wondering why one of my USB 3.0 cards wasn't achieving full speed I finally discovered that PCI Express Slot 3 is NOT PCI Express 2.0. It is 1.1, so it runs at half the speed.
All the other PCI Express slots on the board are 2.0... just not that one. Very weird.
So after pouring through the Intel schematics and docs, I finally figured it out. Others might have already known but it didn't really click until just now.
Our board is a Dual IOH board, which means it has two I/O Hub controllers (the 5520 chipsets). I only thought boards could have one chipset like this, but I guess I learned differently now.
Each Intel 5520 can support 36 PCI Express 2.0 Lanes in x16/x16/x4 configuration. The ICH10R (southbridge) I/O Hub provides an additional PCIe 1.1 interface at x8 width and a traditional PCI bus, not to mention 12x USB 2.0 ports, 6x SATA II, and audio.
On our machines, the gigabit ports are connected to the southbridge I/O hub via PCI Express x1 for each port, and the firewire is attached to the traditional PCI bus.
I found a block diagram in a publication that HP released comparing the Z800 to the Z820. I modified it to add which slot is which on the diagram and am attaching it to this post.
There are a few things I found puzzling about the design:
- Slot #4 should be a full x16. My hardware scanning utilities haven't found any other x8 devices attached to that IOH so I am perplexed as to why HP nerfed a slot to x8 when it didn't really need to. Would it really save that much money? Maybe they couldn't find a way to fit 8 more lanes on the PCB between the IOH and the slot?
- It seems Slot #7 and the SAS Controller share a split x16 on the second IOH. If that's the case, where is the x4 from that hub? Why couldn't they make Slot #3 use these lanes instead of the ones from the southbridge that are slower? I hope the answer is engineering and not cost savings cause that would be aggravating.
- Between the gimped Slot #4 and missing x4 on IOH #2 it seems like we could actually squeeze two more perfectly good PCI Express slots out of this motherboard.... which would be nice cause I currently need them.